Embodiments of the invention relate generally to MR imaging and, more particularly, to a system and method for modeling gradient coil operation induced magnetic field and/or harmonics drift.
In MRI and NMR systems, a number of coils carry an electric current to generate a high strength, relatively homogeneous magnetic field. This field may be referred to the main field or B0 field. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals is digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Rather than being homogenous, the gradient fields (Gx, Gy, and Gz) vary in magnitude along each respective axis. In cylindrical magnet systems, the coils generating the main field are axially aligned. Typically, gradient coils are arranged in a tubular space radially inside of the main field coils. In typical arrangements, the gradient coils comprise resistive wire embedded in a potted material such as a resin.
Passive shimming arrangements commonly employ shim trays that are housed within slots formed in the potting material of the gradient coils, in directions parallel to the magnet axis. The shim trays include a number of pockets along their length. Shim pieces, typically flat square or rectangular pieces of steel, are places within the pocket, and then the shim tray is loaded into the gradient coils.
The operation of the gradient coils commonly involves a series of fast “on-off” switches depending on the pulse sequence is used. It has been observed that the gradient operation can cause B0 field drift and harmonics drift during operation, and image quality can be compromised as a result. Such gradient operation is particularly detrimental to fully passive shimming magnets where large amount of steel shims are used to maintain homogeneity.
The B0 drift and harmonic drift caused by operations of the gradient coils have complex dependence to various components in magnet systems via different mechanisms. FIG. 1 shows a block diagram correlating gradient operation with magnetic field drift and harmonic drift. A first block 2 represents gradient operation that occurs during an MR scan sequence. For example, gradient operation may occur in a spin echo MR sequence where the gradient coils of the MR system are powered according to a pulse sequence such that MR data may be acquired for a particular slice of a patient or scan object. Gradient operation 2 can lead to B0 and harmonic drift 4 by way of a plurality of contributing factors 6 including a warm bore contribution 8, a contribution due to pressure variation 10, and a contribution due to passive shims 12 in an MR system. B0 and harmonic drift 4 can lead to compromised image quality.
Warm bore contribution 8 can be caused, for example, by eddy current heating 14 and other heating mechanisms 16 such as convection and conduction. Pressure variation 10 can be caused, for example, by heat transfer by induced eddy currents from the gradient coils to the helium vessel, which increases pressure in the helium vessel. A temperature change in the helium in the helium vessel may also cause pressure variation 10 to change. Additionally, the passive shims 12 may affect B0 and harmonic drift 4 due to eddy current heating 18 and other heating mechanisms 20.
FIG. 2 is a block diagram showing a contribution to magnetic field drift and harmonic drift via passive shims. The switching electromagnetic fields 22 generated by gradient coils can induce eddy current 24 into the steel, iron, or other ferromagnetic shims that results in Ohmic losses 26, which cause the temperature to rise in the shims 28. In addition the heat transfer from hot gradient surface and gradient cooling water will change the shim temperature and hence the shim magnetization. The shims are commonly magnetized to saturation along the easy axis by the main field and contribute to the B0 field and harmonics in the field-of-view (FOV) as dipoles. The saturation magnetization of soft magnetic material is temperature dependent; therefore, the magnetic field produced by the shims is temperature dependent and can be changed 30. The change in saturation magnetization 30, which can be altered by operation of the gradient coils as shown, contributes to magnetic field drift and harmonic drift 32.
It would therefore be desirable to have a system and method capable of characterizing and compensating for magnetic field drift and/or harmonic drifts for MR imaging and image reconstruction.